The present invention relates to a modified pitch superior for use in the production of carbon fibers having a high strength and a high elastic modulus (Young's modulus).
At present, carbon fibers are prepared mainly from polyacrylonitrile. But the use of polyacrylonitrile is disadvantageous in that it is expensive, the original fibrous form easily gets out of shape at the time of heat carbonization treatment, and the carbonization yield is poor.
Recently, in view of such drawbacks, there have been reported a number of methods for producing carbon fibers from a less expensive pitch. However, carbon fibers obtained from pitch still involve a problem such that they are inferior in strength as compared with polyacrylonitrile carbon fibers.
It has recently been reported (see U.S. Pat. No. 4,005,183) that a carbon fiber superior in both elastic modulus and strength is obtainable by heat-treating a commercially available petroleum pitch to obtain a pitch containing 40 to 90 wt. % of an optically anisotropic liquid crystal called mesophase, then melt spinning the mesophase-containing pitch, rendering the resultant pitch fiber infusible, followed by carbonization and subsequent graphitization if required.
However, since a pitch containing not less than 40 wt. % of mesophase is extremely high in its softening point and viscosity, its melt spinning requires a high temperature usually not lower than 350.degree. C. As a result, the pitch is apt to undergo a thermal decomposition in the course of melt spinning and produce a light gas, thus making it difficult to attain a uniform spinning.
In case the content of the mesophase is adjusted low with a view to adjusting the softening point and viscosity of the resulting pitch, there occurs separation between optically anisotropic and isotropic regions and the melt characteristic of the pitch is greatly deteriorated. More particularly, even if a pitch having a low mesophase content is subjected to melt spinning, there occurs breakage of thread frequently, and in the worst case the resultant fiber is like a linkage of unmelted particles, and even if such a fiber is treated by a conventional method, there is not obtained a carbon fiber having a high strength and a high elastic modulus.